I currently have an erskine fire extinguisher fitted to my formula ford (electrical system) I am going to replace this with an AFFF system soon and I was wondering which is the best way to disconnect the existing system (as I don't want to set it off!!!)

Could we presume that if you isolate the car's battery (remove from car?), we remove the possibility of electrical charge? Or is the Erskin self supporting? Is there a pin? Or an 'armed/disarmed' switch?

Normally extinguisher systems are plumbed in with connections which are released by pushing a plastic or chrome plated collar which sits over the tube where it goes into a connection. Push the collar towards the connection and then pull the pipe away from the connection. That part can be a bit tricky, someone had to show me how before I could make it work.
As the Evil one says, there must be a pin or a hole to put a pin into somewhere, I haven't met that type, but I can't imagine a system without one. Don't forget to take the pin out if you do use it again, the scrutineers get wildly exited if you present a car with the extinguisher pin still in, and even more so if they find it in as you come back off track.

There is no pin in an electrical system what there should be is a small (normaly red) box this will have a switch on it, normaly if it is in the centre it is safe, you can push it up and it will test your battery (as said before it is not run of your car so carry a spare)if you push it down it is armed.